Warpage Control in the Packaging of Integrated Circuits

ABSTRACT

A method includes placing a first package component over a vacuum boat, wherein the vacuum boat comprises a hole, and wherein the first package component covers the hole. A second package component is placed over the first package component, wherein solder regions are disposed between the first and the second package components. The hole is vacuumed, wherein the first package component is pressed by a pressure against the vacuum boat, and wherein the pressure is generated by a vacuum in the hole. When the vacuum in the hole is maintained, the solder regions are reflowed to bond the second package component to the first package component.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. application Ser. No.16/599,959, filed on Oct. 11, 2019, which is a continuation of U.S.application Ser. No. 15/380,671, filed on Dec. 15, 2016, now U.S. Pat.No. 10,512,124, issued on Dec. 17, 2019, which is a divisional of U.S.application Ser. No. 13/559,318, filed on Jul. 26, 2012, now U.S. Pat.No. 9,538,582 issued Jan. 3, 2017, which applications are incorporatedherein by reference.

BACKGROUND

In the packaging of integrated circuits, device dies or packages arepackaged onto package substrates, which include metal connections thatare used to route electrical signals between opposite sides of thepackage substrates. The device dies may be bonded onto one side of apackage substrate using flip chip bonding, and a reflow is performed tomelt the solder balls that interconnect the dies and the packagesubstrate.

The package substrates may use materials that can be easily laminated.In addition, organic materials may be used as the dielectric materialsof the package substrate. These materials, however, are prone to warpagecaused by elevated temperatures used in the reflow of the solder.Furthermore, during the bonding process, since the device dies and thepackage substrates have significantly different Coefficients of ThermalExpansion (CTEs), the warpage in the dies and the package substrates isworsened. For example, the silicon in the device dies has a CTE close toabout 3.2, while the package substrates may have a CTE between about 17and 10, or even higher. The warpage in the package substrates may causeirregular joints and/or bump cracks. As a result, the yield of thepackaging process is adversely affected.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1A and 1B include a top view and a cross-sectional view of apackage substrate strip in accordance with some embodiments;

FIGS. 2A and 2B are cross-sectional views of exemplary vacuum boats inaccordance with exemplary embodiments;

FIG. 2C is a top view of the vacuum boats in FIGS. 2A and 2B;

FIG. 3 illustrates a plurality of package components placed over anotherpackage component; and

FIG. 4 illustrates a cross-sectional of a packaging process, wherein thevacuum boat and the package components that are to be bonded go througha reflow process.

DETAILED DESCRIPTION

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare illustrative, and do not limit the scope of the disclosure.

Methods for forming packages are provided in accordance withembodiments. The variations of the embodiments are discussed. Throughoutthe various views and illustrative embodiments, like reference numbersare used to designate like elements. It is appreciated that theembodiments such as what are shown in FIGS. 1A through 4 are exemplaryembodiments, and more embodiments may be developed based on the teachingof the exemplary embodiments.

FIGS. 1A and 1B illustrate a top view and a cross-sectional view,respectively, of an exemplary package component 10, on which a bondingprocess in accordance with exemplary embodiments is performed. Packagecomponent 10 may be a package substrate strip, and hence is referred toas package substrate strip 10 hereinafter, although package component 10may be another type of package component such as an interposer. Packagesubstrates 12 may be laminate substrates, which include a plurality ofdielectric films laminated together. In accordance with embodiments,package substrate strip 10 includes a plurality of package substrates 12that are identical to each other. In some embodiments, packagesubstrates 12 are distributed uniformly throughout package substratestrip 10, and may have a pattern of an array. In other embodiments,package substrates 12 are disposed as a plurality of groups, with theinter-group spacing between the groups greater than the inner-groupspacing between package substrates 12 that are in the same group.

FIG. 1B schematically illustrates a cross-sectional view of one ofpackage substrates 12, wherein the cross-sectional view is obtained fromthe plane crossing line 1B-1B in FIG. 1A. A plurality of connectors 16,which may be pre-solder regions, metal pads, or non-reflowable metalbumps, is formed on a side of package substrate 12. Metal features 16are electrically coupled to metal features such as bond pads 18 on theopposite side of package substrate 12. Dashed lines 14 represent theelectrical features that couple features 16 to features 18, wherein theelectrical features may include a plurality of metal lines and vias thatinterconnect the metal lines.

FIGS. 2A and 2B illustrate cross-sectional views of vacuum boat 20 inaccordance with exemplary embodiments. Vacuum boat 20 includes base 22,which further includes lower portion 22A, upper portion 22B, andair-penetration layer 24 over and connected to upper portion 22B. Baseportion 22A is underlying and overlapped by air-penetration layer 24,and is vertically spaced apart from air-penetration layer 24 by airpassage 28, which is a space filled with air (or vacuumed). Base portion22B forms a ring that encircles air passage 28. Furthermore, baseportion 22B joins air-penetration layer 24 with no gap therebetween.Base 22 may be formed of non-porous materials that can effectivelyprevent air from penetrating through. Air passage 28 may have a shapethat is similar to the top-view shape (such as a rectangular shape) ofpackage substrate strip 10. In some exemplary embodiments, base portions22A and 22B and air-penetration layer 24 are formed of a rigid materialthat is strong enough to maintain vacuum in air passage 28, so thatvacuum boat 20 is not crushed by the vacuum. For example, base portions22A and 22B and air-penetration layer 24 may be formed of a metal or ametal alloy including copper, aluminum, stainless steel, or the like.Alternatively, base portions 22A and 22B and air-penetration layer 24are formed of a ceramic or ceramics.

Referring again to FIGS. 2A and 2B, base 22 may also include thermalinsulation layer 23, which is formed of a thermal insulating material.Thermal conductivity of thermal insulation layer 23 may be lower thanabout 1 W/K·m, for example. Thermal insulation layer 23 may sustain themelting temperatures (or temperatures slightly higher) of solderswithout being damaged. For example, thermal insulation layer 23 isconfigured to sustain temperatures higher than about 200° C., or higherthan about 230° C. In some exemplary embodiments, thermal insulationlayer 23 is formed of Teflon.

Referring to FIG. 2A, thermal insulation layer 23 has the function ofpreventing the heat absorbed by air-penetration layer 24 to be conductedto base portion 22A during the reflow process shown in FIG. 4. Ifthermal insulation layer 23 is not included, base portion 22A, due toits large volume, may absorb too much heat during the reflow process inFIG. 4, and cause adverse reduction in the temperature of the packagecomponents 12 and 38 and solder regions 40 (FIG. 4).

As shown in FIGS. 2A and 2B, air-penetration layer 24 may include aplurality of holes 26. Therefore, air passage 28 is connected to, andmay exchange air with, external environment (for example, open air)through holes 26. Air passage 28 is also connected to outlet 29, whichmay be connected to external environment through pump 30 and valve 32.Outlet 29 is used to vacuum air passage 28 and holes 26 (when packagesubstrate strip 10 is placed on air-penetration layer 24, as shown inFIGS. 3 and 4) to generate a vacuum environment. Valve 32 is also usedto release vacuum by allowing air to flow into air passage 28 and holes26. In some embodiments, an additional valve (not shown) may beconnected to air passage 28, and hence valve 32 may be used forvacuuming, while the additional valve may be used for releasing thevacuum.

FIG. 2B illustrates a cross-sectional view of vacuum boat 20 inaccordance with alternative embodiments. In these embodiments, thermalinsulation layer 23 is formed as a top portion of air-penetration layer24. Holes 26 are formed in thermal insulation layer 23, and penetrateinto a bottom portion of air-penetration layer 24, which is belowthermal insulation layer 23. The lower portion of air-penetration layer24 may be formed of a rigid material such as a metal, a metal alloy, aceramic, or the like, so that the lower portion provides adequatemechanical support to thermal insulation layer 23. When vacuum boat 20in FIG. 2B is used to bond package substrate strip 10 and packagecomponents 38 (FIGS. 3 and 4), package substrate strip 10 may be incontact with thermal insulation layer 23 in these embodiments, and hencethe heat in package substrate strip 10 and solder regions 40 (FIGS. 3and 4) is not conducted into the base 22 of vacuum boat 20.

FIG. 2C illustrates an exemplary top view of the structure shown inFIGS. 2A and 2B. It illustrates a plurality of holes 26 distributedthroughout air-penetration layer 24. The plurality of holes 26 may bedistributed substantially uniformly, for example, as an array, or in anyother pattern such as a radius pattern, a spiral pattern, or the like.FIG. 2C also illustrates package substrate strip 10 that is to be placedon air-penetration layer 24, wherein package substrate strip 10 isillustrated using dotted lines. Air-penetration layer 24 may have arectangular top-view shape, although other shapes may also be adopted.

In FIGS. 3 and 4, although pump 30 and or valve 32 are illustrated asbeing attached to vacuum boat 20, pump 30 and valve 32 may be connectedto vacuum boat 20 before the reflow process as shown in FIG. 4, anddisconnected from vacuum boat 20 after the reflow process. Furthermore,in FIGS. 3 and 4, the vacuum boat 20 in FIG. 2A is illustrated toexplain the concept of the embodiments. The vacuum boat 20 in FIGS. 3and 4, however, may also adopt the structure shown in FIG. 2B.

Referring to FIG. 3, package substrate strip 10 is placed over vacuumboat 20. Holes 26 are covered by, and sealed by, package substrate strip10. Pump 30 then evacuates the air in air passage 28 and holes 26 togenerate vacuum therein. Arrows 36 represent the direction that the airis evacuated. During the vacuuming, a downward force is applied topackage substrate strip 10 due to the air pressure caused by the vacuum,and hence package substrate strip 10 is flattened. During the vacuuming,the pressure in air passage 28 and holes 26 may be at any value lowerthan an atmosphere. For example, the pressure in air passage 28 may belower than about 50 percent of one atmosphere. Package components 38 areplaced over package substrate 12, for example, with a one-to-onecorrespondence. Solder regions 40 are disposed between packagesubstrates 12 and package components 38. The placement of packagecomponents 38 on package substrate strip 10 may be performed before orafter the vacuuming is started. In some embodiments, package components38 are dies (such as device dies comprising CMOS devices), packages, orthe like. Throughout the description, vacuum boat 20, package substrates12, and package components 38 in combination are referred to as reflowunit 42.

Next, as shown in FIG. 4, reflow unit 42 goes through a reflow process,so that solder regions 40 are reflowed, and hence package components 38are bonded to the respective underlying package substrates 12. In someexemplary embodiments, the reflow is performed using a convection-typereflow apparatus, as shown in FIG. 4. For example, the reflow processmay be performed in chamber 50. Reflow unit 42 is transferred byconveyor belt 44 into and out of chamber 50. During the reflow, conveyorbelt 44 and reflow unit 42 move forward, as indicated by arrows 46. Itis appreciated that other types of reflow methods other than theconvection-type reflow may also be used to perform the reflow, inaccordance with alternative embodiments. As shown in FIG. 4, a pluralityof reflow units 42 may be disposed on conveyor belt 44, and aretransferred into and out of chamber 50 one by one. Reflow units 42 areremoved from conveyor belt 44 after they are transferred out of chamber50, and additional reflow units 42, which include package components 38and package substrates 12 that have not bonded to each other yet, areplaced on conveyor belt 44.

During the reflowing, conveyor belt 44 transfers reflow unit 42 throughone or more heating zones, each including one or more heat sources 48for heating solder regions 40. When reflow unit 42 is transferredpassing heat sources 48, solder regions 40, which are between packagecomponents 38 and the underlying package substrates 12, are heated to atemperature higher than the melting temperature of solder regions 40,and hence solder regions 40 are molten. In some embodiments, heatsources 48 are disposed over and/or under reflow unit 42 (and conveyorbelt 44). Heat sources 48 may be radiation-type heating sources such asinfrared radiant sources, or may be configured to blow hot air to reflowunit 42. The arrows pointed away from heat sources 48 symbolize theradiated heat, the hot air, or the like.

Throughout the reflow process, pump 30 operates to maintain the airpressure on package substrates 12 through the generation of the vacuum.The pumping of air may be started at one of several time points. Forexample, the pumping may be started after package substrate strip 10 isplaced on vacuum boat 20, and before or after the placement of packagecomponents 38. In alternative embodiments, the pumping may also bestarted after the placement of reflow unit 42 on conveyor belt 44, andbefore solder regions 40 are molten. In yet alternative embodiments, thepumping may also be started after the molten of solder regions 40, andbefore the solidification of solder regions 40. The vacuum is maintaineduntil the reflow process ends. By then, solder regions 40 are cooled andat least substantially, or fully, solidify. Package components 38 arethus bonded to the underlying package substrates 12. The vacuum may thenbe released, for example, by allowing air to be conducted into airpassage 28, which may be achieved through valve 32. Pump 30 may then bedisconnected from vacuum boat 20.

In some embodiments, pumps 30 in different reflow units 42 may beseparate pumps. In alternative embodiments, the illustrated pumps 30 maybe the same pump that has a plurality of inlets, each connected to oneof reflow units 42.

In the embodiments, during the reflow process, package substrates 12 areflattened by the pressure caused by the vacuum in air passage 28 andholes 26 (FIGS. 3 and 4). Accordingly, the warpage of package substrates12, which warpage is caused by the difference in Coefficients of ThermalExpansion (CTEs) of different materials in package components 38 andpackage substrates 12, is substantially eliminated. The reliability ofthe resulting packages is improved.

In accordance with embodiments, a method includes placing a firstpackage component over a vacuum boat, wherein the vacuum boat comprisesa hole, and wherein the first package component covers the hole. Asecond package component is placed over the first package component,wherein solder regions are disposed between the first and the secondpackage components. The hole is vacuumed, wherein the first packagecomponent is pressed by a pressure against the vacuum boat, and whereinthe pressure is generated by a vacuum in the hole. When the vacuum inthe hole is maintained, the solder regions are reflowed to bond thesecond package component to the first package component.

In accordance with other embodiments, a method includes placing apackage substrate strip over a vacuum boat, wherein the vacuum boatincludes a plurality of holes, and wherein the package substrate stripcovers the plurality of holes. A plurality of package components isplaced over the package substrate strip, wherein each of the pluralityof package components is placed over one of package substrates in thepackage substrate strip. Air is pumped out of the plurality of holes togenerate a pressure on the package substrate strip, wherein the pressurepresses the package substrate strip against the vacuum boat. Solderregions between the plurality of package components and the packagesubstrates are reflowed to bond the plurality of package components tothe package substrates, wherein the step of pumping air is performeduntil the solder regions substantially solidify.

In accordance with yet other embodiments, a vacuum boat includes a basethat includes an upper portion forming a ring, and a lower portion underthe upper portion, wherein the upper portion is connected to edges ofthe lower portion. The vacuum boat further includes an air-penetrationlayer over and connected to the upper portion of the base. Theair-penetration layer, the upper portion, and the lower portion enclosean air passage therebetween. A plurality of holes penetrates through theair-penetration layer, wherein the plurality of holes connects the airpassage to an external environment. A thermal insulation layer isdisposed over the lower portion of the base. An outlet is connected tothe air passage.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: placing a package substratestrip over a vacuum boat, wherein the vacuum boat comprises: anair-penetration layer; and a thermal insulation layer over theair-penetration layer, wherein the package substrate strip is in contactwith the thermal insulation layer, and the package substrate stripcovers a plurality of through-holes that penetrate through theair-penetration layer and the thermal insulation layer; vacuuming theplurality of through-holes; and performing a reflow process to reflowsolder regions over the package substrate strip while vacuuming theplurality of through-holes.
 2. The method of claim 1 further comprisingplacing a plurality of package components over the package substratestrip, wherein after the reflow process, the solder regions join thepackage substrate strip with the plurality of package components.
 3. Themethod of claim 1, wherein vacuuming the plurality of through holescomprises vacuuming an inner space in the vacuum boat, with the innerspace being connected to the plurality of through-holes.
 4. The methodof claim 1, wherein vacuuming the plurality of through-holes ismaintained until the solder regions solidify after the reflow process.5. The method of claim 1, wherein the reflow process is performed usinga convection-type reflow process with the vacuum boat being transferredon a conveyor belt.
 6. The method of claim 5, wherein when the vacuumboat is transferred on the conveyor belt, a pump is operated to vacuumthe plurality of through-holes.
 7. The method of claim 1, wherein theplurality of through-holes is grouped into a plurality of groups, withinner-group spacing between through-holes in a same group being smallerthan inter-group spacing between neighboring groups of the plurality ofgroups.
 8. A method comprising: placing a first package component over avacuum boat, wherein the vacuum boat comprises: a thermal insulationlayer in physical contact with the first package component; anair-penetration layer below the thermal insulation layer; and a baseportion underlying and overlapped by the air-penetration layer, whereinan air-passage is disposed between the base portion and theair-penetration layer, and wherein holes that extend through the thermalinsulation layer and the air-penetration layer are connected to andexchange air with the air-passage; decreasing pressure in theair-passage to be lower than one atmosphere; placing a plurality ofsecond package components over the first package component; and when thepressure in the air-passage is lower than one atmosphere, reflowingsolder regions to couple the plurality of second package components tothe first package component.
 9. The method of claim 8, whereindecreasing pressure in the air-passage comprises connecting a pump toand vacuuming the air-passage.
 10. The method of claim 9, wherein avacuum in the air-passage is maintained until the reflowing the solderregions is complete.
 11. The method of claim 9, wherein the vacuuming ofthe air-passage is started after placing the plurality of second packagecomponents over the first package component.
 12. The method of claim 9,wherein the vacuuming of the air-passage is started before placing theplurality of second package components over the first package component.13. The method of claim 8, wherein during the reflowing the solderregions, the vacuum boat, the first package component, and the pluralityof second package components are moved by a conveyor belt.
 14. Themethod of claim 8, wherein a material of the thermal insulation layer isdifferent from a material of the air-penetration layer.
 15. A methodcomprising: placing a package substrate strip over a vacuum boat,wherein the vacuum boat comprises: a rigid layer, wherein the rigidlayer comprises a plurality of through-holes that penetrate through therigid layer; a first portion vertically spaced apart from the rigidlayer by an air passage, wherein the air passage is connected to theplurality of through-holes; a second portion that comprises a ring,wherein the second portion encircles the air passage and is in physicalcontact with the rigid layer; and a thermal insulation layer comprisinga ceramic; connecting a valve and a pump to the vacuum boat in series,the pump being between the valve and the vacuum boat; evacuating airfrom the plurality of through-holes and the air passage using the pump;placing a plurality of package components over the package substratestrip; and conveying the vacuum boat, the package substrate strip, andthe plurality of package components through a heating zone.
 16. Themethod of claim 15, wherein the thermal insulation layer is disposedabove the air passage.
 17. The method of claim 15, wherein the thermalinsulation layer is disposed below the air passage.
 18. The method ofclaim 15, wherein during conveying the vacuum boat, the packagesubstrate strip, and the plurality of package components through theheating zone, solder regions are reflowed to join the plurality ofpackage components to the package substrate strip.
 19. The method ofclaim 18, wherein during conveying the vacuum boat, the packagesubstrate strip, and the plurality of package components through theheating zone, the reflowing of the solder regions is performed using aconvection-type reflow process, with the vacuum boat being transferredon a conveyor belt.
 20. The method of claim 15, wherein the thermalinsulation layer is in physical contact with a top surface of the rigidlayer.